Spectacle lenses are typically manufactured to provide optimum clarity of imaging in the monocular condition. Spectacle lenses manufactured this way can cause vision problems such as aniseikonia and anisophoria for individuals with anisometropia, a condition in which the two eyes have unequal refractive power.
Aniseikonia is a term used to describe and quantify the variance of ocular image sizes in a pair of eyes. All spectacle lenses distort the image on the viewer's retina. Lenses that correct near-sightedness cause the viewed object to appear smaller while lenses that correct far-sightedness cause the object to appear larger. Lenses that correct astigmatism cause a fattening or thinning of the object. Eyeglasses also distort the perceived position of an object due to the fact that they remain stationary while the eye moves behind them. If the prescription for each eye is about the same then both eyes receive about the same distortion and there are few adaptation problems. It is when there is a difference in the prescription that discrepancies in size and position occur with the associated dysfunction. Optometrists call ocular image size discrepancies sensory aniseikonia and position discrepancies motor aniseikonia or anisophoria.
The effects of aniseikonia and anisophoria may include ocular discomfort, reduced depth perception, poor reading skills, amblyopia, learning disability, gross motor skill dysfunction, risk of falling, dipolpia or double vision that increases with increased eccentricity of gaze (i.e. it gets worse if you look further off centre), inability to fuse the images, distortion of floor (which may seem to be raised or lowered), suppression of the vision in one eye, headaches and general vision malaise, impaired vision, nausea and dizziness.
Bicentric lens designs (slab-off) provide a discrete prismatic correction to the reading portion of one spectacle lens in a pair to ameliorate the anisophoria in that area. But bicentric lenses are not an option with digitally surfaced lenses because at present there is no capability to manufacture a bicentric lens with digital technology surfacing. Bicentric lenses also fail to attend to the correction of the anisophoria in all positions of gaze or inter-ocular spectacle magnification.
Accordingly, there remains a need for improvements in the art.